1. Field of the Invention
The invention relates to a system for transmitting a data signal at a symbol rate 1/T through a noisy dispersive channel to a data receiver, said channel introducing intersymbol interference and noise into the transmitted data signal; and said receiver estimating the most likely sequence of transmitted data symbols by keeping track of candidate data sequences that are recursively updated on the basis of likelihood measures which are determined with the help of means for estimating hypothesized channel outputs in the absence of noise.
2. Description of the Related Art
Such a system is known from an article by G. D. Forney, Jr., entitled "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference", IEEE Trans. Inform. Theory, Vol. IT-18, No. 3, pp. 363-378, May 1972.
In the forthcoming decades, digital optical storage is expected to find widespread use for storage of computer data as well as of digitized audio and video signals. In systems of this type, binary information is stored as a sequence of pits and lands in an optical medium. Imperfections in the writing process may cause pits and lands to differ in shape or length. In the replay process, this asymmetry manifests itself in the form of nonlinear intersymbol interference (ISI). This disturbance comes on top of linear ISI that arises due, for example, to limitations of optical resolution, and to noise generated in e.g. laser diodes and preamplifiers. Conventional reception techniques are, in general, only able to deal with the latter two imperfections. This even applies for the most powerful variety, whose representatives are commonly referred to as Viterbi detectors. Conventional Viterbi detectors form an estimate of the most likely transmitted data sequence, assuming that only linear ISI and noise are present. To this end, they maintain a list of candidate data sequences that are referred to as survivors. These survivors are recursively extended, and a selection process takes place on the basis of likelihood measures that are calculated for each survivor by comparing the actual channel output signal with a hypothesized output signal that would result if noise were absent and the concerned survivor would have been transmitted. The aforementioned means to form these hypothesized channel output signals conventionally consist of linear weighing networks that operate on a given number of the most recent symbols of concerned survivors.
Basic elements of this detection process are described in more detail, for example, in the aforementioned article by Forney. The conventional Viterbi detector developed in this article has the disadvantage that its complexity grows rapidly to unmanageable proportions as channel memory lengths increase. To overcome this problem, various simplified versions of the conventional Viterbi detector have been developed in recent years, as exemplified, for example, by an article by J. W. M. Bergmans, S. A. Rajput and F. A. M. van de Laar entitled "On the Use of Decision Feedback for Simplifying the Viterbi Detector", Philips J. Res., Vol. 42, No. 4, pp. 399-428, 1987. Partly as a consequence of these efforts towards simplification, conventional Viterbi detectors now find application in such areas as voiceband modems and digital magnetic recording. To date, however, nonlinear ISI has discouraged their use in optical storage.
An article by M. F. Mesiya, P. J. McLane, and L. L. Campbell entitled "Maximum Likelihood Sequence Estimation of Binary Sequences Transmitted over Nonlinear Channels", IEEE Trans. Commun., Vol. COM-25, No. 7, pp. 633-643, July 1977, puts forth a novel type of Viterbi detector that distinguishes itself from conventional Viterbi detectors in that it can handle nonlinear ISI. Unfortunately this ability comes, in general, at the cost of a greatly increased complexity.